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Section 1: Introduction

Plants are approximately 90% water. The remaining dry matter is mostly composed of the elements listed below. The elements carbon (C), hydrogen (H) and oxygen (O) comprise 89% of the dry matter of most plants. The plant is able to derive these 3 elements from water and the atmosphere. The other essential elements constitute only about 1% of the total plant weight and about 11% of the dry matter. However, these are the elements (referred to as mineral elements or mineral nutrients) that need to be supplied in the substrate components or fertilization program.

These elements can be broken into 3 major categories. The first is referred to as primary macroelements because the plant needs relatively large amounts of these elements. The primary macroelements include nitrogen (N), phosphorus (P), and potassium (K). The second category is the secondary macroelements and includes calcium (Ca), magnesium (Mg) and sulfur (S). The third category is the microelements. These are required in much smaller amounts but are no less important than the primary and secondary macroelements. The microelements include iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), chlorine (Cl), and nickel (N). There are several other elements that are qualified as beneficial including silicon (Si), cobalt (Co) and sodium (Na).  Although beneficial, these elements are not considered essential since they are not required for all plants to grow and complete their life cycle.

The goal of a greenhouse fertilization program is to provide these mineral elements to the crop at optimal concentrations and ratios to each other. Mineral elements may be provided in various forms including salts, oxides and chelates. The terms oxides and chelates will be discussed more under the micronutrient section of this learning unit. However, before discussing the macroelements it is important to understand the term “fertilizer salt”. A salt is a compound that is generally water-soluble (dissolves in water) and when placed in water dissociates (breaks apart) into an anion (negatively charged particle) and a cation (positively charged particle).  For example, common table salt is sodium chloride or NaCl. When placed in water it dissolves by dissociating into Na⁺ and Cl^-. Many, although not all, of the fertilizers that are used to provide the macroelements (and often many microelements) are salts (thus we call them fertilizer salts). Some examples are shown below with the fertilizer salt and its respective anion and cation. Therefore, when added to water, fertilizer salts usually readily dissolve and dissociate into their respective anions and cations. This is important because this allows for these water-soluble fertilizer salts to be used to formulate water-soluble fertilizers to be used in liquid fertilization programs in the greenhouse.

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